A technique that is now widely used for insulating electrical conductors such as wires, and for producing coatings for other purposes and on various substrates, entails the exposure of the grounded workpiece to a cloud of electrostatically charged particles, thereby causing the particles to deposit thereupon for subsequent integration. Typical equipment used for that purpose is disclosed and claimed in the following U.S. Pat. Nos. 3,828,729 to Goodridge, 3,916,826 to Knudsen, 4,030,446 to Karr, 4,297,386 and 4,330,567 to Gillette, 4,332,835 to Knudsen, 4,418,642 and 4,472,452 to Gillette et al, and 4,517,219 to Hajek. Electrostatic fluidized bed apparatus and systems that are highly effective for such coating are commercially available from the common assignee of those patents and of the instant application, Electrostatic Technology Incorporated, of New Haven, Conn.
Dunford et al application Ser. No. 708,989, which will issue on Aug. 19, 1986 as U.S. Pat. No. 4,606,928 and is also of common assignment herewith, discloses and claims a method, apparatus, and system by which workpieces, and particularly conductors of continuous length, can be coated by electrostatic powder deposition, quickly, efficiently, safely, and with an exceptionally high degree of uniformity in the build. As is true of other electrostatic fluidized bed coating equipment, that of the Dunford et al patent employs a planar porous plate above which a cloud of charged particles is produced. In accordance therewith, however, a secondary, generally tubular cloud of charged particles is produced within the primary cloud, from which the particles move radially to coat the workpiece as it is conveyed therethrough.
Despite the highly desirable results that are achieved by use of the Dunford et al invention, as well as by use of certain of the other prior art methods and apparatus, a number of disadvantages are inherent therein. For example, such units tend to be relatively large, and to require a considerable volume of coating powder for proper operation; this in turn means that a relatively large capacity recovery and dust collection subsystem must be employed. The coating material within the coating chamber of such a unit must not only be kept at a fairly constant level during operation (necessitating the provision of a level control arrangement), but it must also be removed at night or during other similar periods of nonuse, to avoid moisture pick-up problems.
In addition, even though measures may be taken to design the apparatus so as to minimize the presence of structure from which collected powder can drop upon the workpiece, and thereby produce flaws in the coating, as a practical matter it is not feasible to eliminate such surfaces entirely. And finally, despite the substantial decreases in voltage requirements enabled by the equipment of the above-mentioned patents, with the concomitant energy savings and enhancements of safety that result, further power reductions would of course be of great benefit.
Accordingly, it is a broad object of the present invention to provide a novel method, apparatus, and system by which workpieces, and particularly conductors of continuous length, can be coated by electrostatic powder deposition, quickly, efficiently, safely, and with an exceptionally high degree of uniformity in the build.
A more specific object of the invention is to provide such a method, apparatus and system in which the coating unit is smaller than prior art apparatus of comparable effectiveness and efficiency, requires less coating powder and a recovery system which is of correspondingly reduced capacity, obviates any need for powder bed level control, and avoids any tendency for powder buildup on surfaces over the workpiece travel path.
It is also an object of the invention to provide such a method, apparatus and system wherein coating can be carried out at voltage levels that are significantly reduced from those heretofore employed for practical high-speed operation, thereby further enhancing safety.
Another object of the invention is to provide such a novel method, apparatus and system wherein the nature of the coating can readily be controlled by the speed of the workpiece and the magnitude of the voltage applied, is highly tolerant of changes of workpiece position within the cloud of charged particles, and is virtually unaffected by normal fugitive electrical effects, such as noise and static.
Still another object of the invention is to provide such a method, apparatus and system wherein economy of production is maximized by the significant reduction of waste produced during start-up and discontinuances of operation.
A still further object of the invention is to provide a novel coating unit which is relatively uncomplicated and inexpensive to manufacture and operate.